Some aircraft are now fitted with an inerting system in which gaseous nitrogen (GN.sub.2) is supplied to the fuel tanks for diluting the oxygen content therein to prevent explosion or combustion of fuel vapor within the tanks. The supply of nitrogen is carried on board in liquid form (LN.sub.2) within a dewar which must be refilled from a supply dewar from time to time under conditions and in a manner so that the LN.sub.2 will boil at a predetermined pressure, such as 65 PSIA, for proper supply to the aircraft inerting system. Prior methods and apparatus for filling the aircraft dewars have not been entirely satisfactory because of complexity and also because during the filling process nitrogen is lost to atmosphere through overboard vents and an excessive amount of heat is added to the LN.sub.2 remaining in the supply dewar.
Because boiling pressure of LN.sub.2 in a supply dewar should be lower than that in a dewar to be filled, it is the practice to deliver LN.sub.2 to an airport main supply dewar at a boiling pressure of about 15 PSIA, at which the temperature is about -320.degree.F (-195.degree.C), for ultimate use in filling the aircraft dewars to the required boiling pressure of 65 PSIA, at which the temperature is about -290.degree.F (-178.degree.C). The stored LN.sub.2 gradually gains heat from its surroundings so that both the temperature and boiling pressure rise. However, the airport main supply is quite large so that the temperature and pressure rise is very gradual.
From the main airport supply, LN.sub.2 is first transferred to a dewar in a vehicle referred to as a ground servicing unit (GSU), which may hold enough LN.sub.2 to fill eight or ten aircraft dewars. When freshly filled a GSU dewar will contain the LN.sub.2 fairly close to 15 PSIA. Then when the LN.sub.2 from the GSU dewar is transferred to an aircraft dewar heat must be added to the transferred fluid to bring the boiling pressure up to 65 PSIA. Any excess heat added to the LN.sub.2 returns to the GSU dewar during the filling process and thus needlessly raising its temperature and boiling pressure. Because of this returned heat and the heat that is constantly being absorbed from its surroundings, the temperature of LN.sub.2 in a GSU dewar may vary considerably from one filling of an aircraft dewar to another. Consequently, the amount of heat to be added during one filling operation can vary considerably from that to be added in a different filling operation.
One might expect the vapor pressure in the GSU dewar ullage to coincide with the boiling pressure of the LN.sub.2 therein and be a measure of the heat in the LN.sub.2 . However, under quiescent conditions heat that has entered the dewar tends to stratify in the ullage so that a reading of ullage pressure in present systems is not a true measure of the heat in the LN.sub.2 and therefore is not a reliable indicator for determining how much heat to add during the filling operation.